Recording media of various types, e.g., magnetic, optical, magneto-optical (“MO”), read-only memory (“ROM”), readable compact disks (“CD-R”), and readable-writable compact disks (“CD-RW”) are widely used in various applications, e.g., in hard disk form, particularly in the computer industry for storage and retrieval of large amounts of data/information. Typically, such media types require pattern formation in the major surface(s) thereof for facilitating operation thereof. For example, magnetic and magneto-optical (MO) recording disks require formation of servo patterns for positioning the read-write transducer over a particular band or region of the media; ROM disks require formation of memory patterns therein; and CD-R and CD-RW disks require formation of wobble groove patterns therein.
Magnetic and magneto-optical (MO) recording media are conventionally fabricated in thin film form; the former are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation (i.e., parallel or perpendicular) of the magnetic domains of the grains of the magnetic material constituting the active magnetic recording layer, relative to the surface of the layer.
In operation of magnetic media, the magnetic layer is locally magnetized by a write transducer or write head to record and store data/information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When the local magnetic field applied by the write transducer is greater than the coercivity of the recording medium layer, then the grains of the polycrystalline magnetic layer at that location are magnetized. The grains retain their magnetization after the magnetic field applied by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The pattern of magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored medium to be read.
A typical contact start/stop (CSS) method employed during use of disk-shaped recording media, such as the above-described thin-film magnetic recording media, involves a floating transducer head gliding at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between mutually sliding surfaces of the transducer head and the disk. During reading and recording (writing) operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the transducer head is freely movable in both the circumferential and radial directions, thereby allowing data to be recorded and retrieved from the disk at a desired position in a data zone.
Adverting to FIG. 1, shown therein, in simplified, schematic plan view, is a magnetic recording disk 30 (of either longitudinal or perpendicular type) having a data zone 34 including a plurality of servo tracks, and a contact start/stop (CSS) zone 32. A servo pattern 40 is formed within the data zone 34, and includes a number of data track zones 38 separated by servo tracking zones 36. The data storage function of disk 30 is confined to the data track zones 38, while servo tracking zones 36 provide information to the disk drive which allows a read/write head to maintain alignment on the individual, tightly-spaced data tracks.
Although only a relatively few of the servo tracking zones are shown in FIG. 1 for illustrative simplicity, it should be recognized that the track patterns of the media contemplated herein may include several hundreds of servo zones to improve head tracking during each rotation of the disk. In addition, the servo tracking zones need not be straight radial zones as shown in the figure, but may instead comprise arcs, intermittent zones, or irregularly-shaped zones separating individual data tracks.
In conventional hard disk drives, data is stored in terms of bits along the data tracks. In operation, the disk is rotated at a relatively high speed, and the magnetic head assembly is mounted on the end of a support or actuator arm, which radially positions the head on the disk surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a circular path on the disk, i.e., over a data track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above that track. By moving the actuator arm, the magnetic head assembly is moved radially on the disk surface between tracks. Many actuator arms are rotatable, wherein the magnetic head assembly is moved between tracks by activating a servomotor which pivots the actuator arm about an axis of rotation. Alternatively, a linear actuator may be used to move a magnetic head assembly radially inwardly or outwardly along a straight line.
As has been stated above, to record information on the disk, the transducer creates and applies a highly concentrated magnetic field in close proximity to the magnetic recording medium. During writing, the strength of the concentrated magnetic field directly under the write transducer is greater than the coercivity of the recording medium, and grains of the recording medium at that location are magnetized in a direction which matches the direction of the applied magnetic field. The grains of the recording medium retain their magnetization after the magnetic field is removed. As the disk rotates, the direction of the writing magnetic field is alternated, based on bits of the information being stored, thereby recording a magnetic pattern on the track directly under the write transducer.
On each track, eight “bits” typically form one “byte” and bytes of data are grouped as sectors. Reading or writing a sector requires knowledge of the physical location of the data in the data zone so that the servo-controller of the disk drive can accurately position the read/write head in the correct location at the correct time. Most disk drives use disks with embedded “servo patterns” of magnetically readable information. The servo patterns are read by the magnetic head assembly to inform the disk drive of track location. In conventional disk drives, tracks typically include both data sectors and servo patterns and each servo pattern typically includes radial indexing information, as well as a “servo burst”. A servo burst is a centering pattern to precisely position the head over the center of the track. Because of the locational precision needed, writing of servo patterns requires expensive servo-pattern writing equipment and is a time consuming process.
An approach for overcoming, or at least alleviating, the above problems associated with writing of magnetic patterns in a magnetic layer, e.g., servo patterns, is disclosed in commonly assigned U.S. Pat. No. 5,991,104 to Bonyhard, the entire disclosure of which is incorporated herein by reference thereto. According to this approach, a method for forming a magnetic transition pattern, such as a servo pattern, in a layer of a magnetic material comprises steps of:
1) aligning a magnetic disk immediately adjacent a master servo-writer medium, the latter constituted of a magnetic layer having a greater agnetic coercivity than the former, wherein the servo-writer medium has a master servo pattern magnetically stored thereon which defines a plurality of concentric tracks;
2) applying a magnetic assist field to the aligned master servo-writer medium and magnetic disk, the magnetic assist field having a substantially equal magnitude at all tracks on the aligned master servo-writer medium and magnetic disk; and
3) rotating the aligned master servo-writer medium and magnetic disk relative to the magnetic assist field.
However, the above-described method incurs several drawbacks associated with its implementation in an industrially viable manner. Specifically, a “one-of-a-kind” master writer with a very high write field gradient is necessary for writing the requisite high intensity, master magnetic servo pattern onto the master disk, and a complicated means for rotating the aligned master servo-writer disk and “slave” workpiece magnetic disk is required, as is a complex system for controlling/regulating/rotating the intensity (i.e., magnitude) and directions of the magnetic assist field.
Commonly assigned, co-pending U.S. patent application Ser. No. 10/082,178, filed Feb. 26, 2002, the entire disclosure of which is incorporated herein by reference, discloses an improvement over the invention disclosed in the aforementioned commonly assigned U.S. Pat. No. 5,991,104, and is based upon the discovery that very sharply defined magnetic transition patterns can be reliably, rapidly, and cost-effectively formed in a magnetic medium containing a longitudinal or perpendicular type magnetic recording layer without requiring expensive, complicated fabrication of a master disk.
Specifically, the invention disclosed in U.S. patent application Ser. No. 10/082,178 is based upon recognition that a stamper/imprinter (analogous to the aforementioned “master”) comprised of a magnetic material having a high saturation magnetization, Bsat, i.e., Bsat≧about 0.5 Tesla, and a high permeability, μ, i.e., μ≧about 5, e.g., selected from Ni, NiFe, CoNiFe, CoSiFe, CoFe, and CoFeV, can be effectively utilized as a contact “stamper/imprinter” for contact “imprinting” of a magnetic transition pattern, e.g., a servo pattern, in the surface of a magnetic recording layer of a magnetic medium (“workpiece”), whether of longitudinal or perpendicular type. A key feature of this invention is the use of a stamper/imprinter having an imprinting surface including a topographical pattern, i.e., comprised of projections and depressions, corresponding to a desired magnetic transition pattern, e.g., a servo pattern, to be formed in the magnetic recording layer. An advantage afforded by the invention is the ability to fabricate the topographically patterned imprinting surface of the stamper/imprinter, as well as the substrate or body therefor, of a single material, as by use of well-known and economical electro-forming techniques (described below in more detail).
According to this invention, the magnetic domains of the magnetic recording layer of the workpiece are first unidirectionally aligned (i.e., “erased” or “initialized”), as by application of a first external, unidirectional magnetic field Hinitial of first direction and high strength greater than the saturation field of the magnetic recording layer, typically≧2,000 and up to about 20,000 Oe. The imprinting surface of the stamper/imprinter is then brought into intimate (i.e., touching) contact with the surface of the magnetic recording layer. With the assistance of a second externally applied magnetic field of second, opposite direction and lower but appropriate strength Hre-align, determined by Bsat/μ of the stamper material (typically≧100 Oe, e.g., from about 2,000 to about 4,500 Oe), the alignment of the magnetic domains at the areas of contact between the projections of the imprinting surface of the stamper/imprinter (in the case of perpendicular recording media, as schematically illustrated in FIG. 2) or at the areas facing the depressions of the imprinting surface of the stamper/imprinter (in the case of longitudinal recording media, as schematically illustrated in FIG. 3) and the magnetic recording layer of the workpiece is selectively reversed, while the alignment of the magnetic domains at the non-contacting areas (defined by the depressions in the imprinting surface of the stamper/imprinter) or at the contacting areas, respectively, is unaffected, whereby a sharply defined magnetic transition pattern is created within the magnetic recording layer of the workpiece to be patterned which essentially mimics the topographical pattern of projections and depressions of the imprinting surface. According to the invention, high Bsat and high μ materials are preferred for use as the stamper/imprinter in order to: (1) avoid early magnetic saturation of the stamper/imprinter at the contact points between the projections of the imprinting surface and the magnetic recording layer, and (2) provide an easy path for the magnetic flux lines which enter and/or exit at the side edges of the projections.
Another process which has been recently studied and developed as a low cost alternative technique for fine dimension pattern/feature formation in a substrate surface is thermal imprint lithography. A typical thermal imprint lithographic process for forming nano-dimensioned patterns/features in a substrate surface is illustrated with reference to the schematic, cross-sectional views of FIGS. 4(A)–4(D).
Referring to FIG. 4(A), shown therein is a stamper/imprinter 10 including a main (or support) body 12 having upper and lower opposed surfaces, with an imprinting layer 14 formed on the lower opposed surface. As illustrated, stamper/imprinter 14 includes a plurality of features 16 having a desired shape or surface contour. A workpiece 18 carrying a thin film layer 20 on an upper surface thereof is positioned below, and in facing relation to the molding layer 14. Thin film layer 20, e.g., of polymethylmethacrylate (PMMA), may be formed on the substrate/workpiece surface by any appropriate technique, e.g., spin coating.
Adverting to FIG. 4(B), shown therein is a compressive molding step, wherein stamper/imprinter 10 is pressed into the thin film layer 20 in the direction shown by arrow 22, so as to form depressed, i.e., compressed, regions 24. In the illustrated embodiment, features 16 of the imprinting layer 14 are not pressed all of the way into the thin film layer 20 and thus do not contact the surface of the underlying substrate 18. However, the top surface portions 24a of thin film 20 may contact depressed surface portions 16a of imprinting layer 14. As a consequence, the top surface portions 24a substantially conform to the shape of the depressed surface portions 16a, for example, flat. When contact between the depressed surface portions 16a of imprinting layer 14 and thin film layer 20 occurs, further movement of the imprinting layer 14 into the thin film layer 20 stops, due to the sudden increase in contact area, leading to a decrease in compressive pressure when the compressive force is constant.
FIG. 4(C) shows the cross-sectional surface contour of the thin film layer 20 following removal of stamper/imprinter 10. The imprinted thin film layer 20 includes a plurality of recesses formed at compressed regions 24 which generally conform to the shape or surface contour of features 16 of the molding layer 14. Referring to FIG. 4(D), in a next step, the surface-imprinted workpiece is subjected to processing to remove the compressed portions 24 of thin film 20 to selectively expose portions 28 of the underlying substrate 18 separated by raised features 26. Selective removal of the compressed portions 24 may be accomplished by any appropriate process, e.g., reactive ion etching (RIE) or wet chemical etching.
The above-described imprint lithographic processing is capable of providing sub-micron-dimensioned features, as by utilizing a stamper/imprinter 10 provided with patterned features 16 comprising pillars, holes, trenches, etc., by means of e-beam lithography, RIE, or other appropriate patterning method. Typical depths of features 16 range from about 5 to about 200 nm, depending upon the desired lateral dimension. The material of the imprinting layer 14 is typically selected to be hard relative to the thin film layer 20, the latter comprising a thermoplastic material which is softened when heated. Thus, suitable materials for use as the imprinting layer 14 include metals, dielectrics, semiconductors, ceramics, and composite materials. Suitable materials for use as thin film layer 20 include thermoplastic polymers which can be heated to above their glass temperature, Tg, such that the material exhibits low viscosity and enhanced flow.
Referring now to FIG. 5, schematically illustrated therein, in simplified cross-sectional view, is a sequence of processing steps for performing nano-imprint lithography of a metal-based workpiece, e.g., a disk-shaped substrate for a hard disk recording medium, utilizing a stamper/imprinter with a lubricated imprinting surface, as disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 09/946,939, filed Sep. 6, 2001, the entire disclosure of which is incorporated herein by reference.
In a preliminary step according to the method, a thin film of a thermoplastic polymer, e.g., polymethylmethacrylate (PMMA), is spin-coated on the substrate surface. In another preliminary step, a stamper/imprinter, e.g., formed of Ni, having an imprinting surface with a negative image of servo pattern features having a lateral dimension of about 600 nm and a height of 170 nm is fabricated by conventional optical lithographic patterning/etching techniques and provided with a thin layer of an anti-sticking or release agent. In the next steps according to the disclosed invention, the system of substrate/workpiece and Ni-based stamper/imprinter is heated to above the glass transition temperature (Tg) of the PMMA, i.e., above about 105° C., and the negative image of the desired pattern on the imprinting surface of the stamper/imprinter is embossed into the surface of the thermoplastic PMMA layer at a pressure of about 10 MPa. The stamper/imprinter is then maintained in contact with the PMMA layer and under pressure until the system cools down to about 70° C., and then removed from the substrate/workpiece to leave replicated features of the imprinting surface in the surface of the PMMA layer. Subsequent processing of the imprinted substrate/workpiece involves selective removal of substrate material utilizing the imprinted layer of thermoplastic material as a pattern defining (etching) mask, followed by removal of the imprinted layer of thermoplastic material.
Still another process which has been recently studied and developed as a low cost alternative technique for fine dimension pattern/feature formation in a substrate surface is imprinting of a sol-gel layer on a substrate surface, as for example, disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 09/852,084, filed May 10, 2001, the entire disclosure of which is incorporated herein by reference.
According to the process disclosed therein, problems attendant upon the use of very hard surfaced, high modulus materials, e.g., of glass, ceramics, or glass-ceramic composites, as substrates in the manufacture of hard disk recording media are addressed, and the invention is based upon the discovery that the surfaces of such materials may be modified, i.e., reduced in hardness, so as to facilitate formation of servo patterns therein, as by a simple and conveniently performed embossing process. According to the invention, modification (i.e., reduction) of surface hardness of high modulus substrates for use in the manufacture of thin film recording media is obtained by first forming a relatively soft sol-gel coating layer on the substrate surface, embossing the desired servo pattern in the exposed upper surface of the relatively soft sol-gel layer utilizing a stamper/imprinter with an appropriately patterned imprinting surface comprising a patterned plurality of depressions and protrusions having a negative image of the desired servo pattern, and then converting the embossed, relatively soft sol-gel layer to a relatively hard glass-like layer while retaining the embossed servo pattern therein. The thus-formed substrate with embossed servo pattern in the exposed surface thereof is then subjected to thin film deposition thereon for forming the layer stack constituting the magnetic recording medium. The inventive methodology advantageously provides servo-patterned recording media without requiring servo-writing subsequent to media fabrication.
Stampers/imprinters for use in a typical application, e.g., servo pattern formation in the recording layer of a disk-shaped, thin film, longitudinal or perpendicular magnetic recording medium comprise an imprinting surface having topographical features consisting of larger area data zones separated by smaller areas with well-defined patterns of projections and depressions corresponding to conventionally configured servo sectors, as for example, disclosed in the aforementioned commonly assigned U.S. Pat. No. 5,991,104. For example, a suitable topography for forming the servo sectors may comprise a plurality of projections having a height in the range from about 20 to about 500 nm, a width in the range from about 0.01 to about 1 μm, and a spacing of at least about 0.01 μm.
Stampers/imprinters suitable for use in performing the foregoing patterning processes may be manufactured by a sequence of steps as schematically illustrated in FIG. 6, which steps include providing a “master” comprised of a substantially rigid substrate with a patterned layer of a resist material thereon, the pattern comprising a plurality of projections and depressions corresponding (in positive or negative image form, as necessary) to the desired pattern to be formed in the surface of the stamper/imprinter. Stampers/imprinters are made from the “master” by initially forming a thin, conformal layer of an electrically conductive material over the patterned resist layer and then electroforming a substantially thicker (“blanket”) metal layer on the thin layer of electrically conductive material, which electroformed blanket layer replicates the surface topography of the resist layer. Upon completion of the electroforming process, the stamper/imprinter is separated from the “master”, which is then re-used for making additional stampers/imprinters.
As indicated above, processes for manufacturing stampers/imprinters for use in optical and magnetic disk pattern replication involve the transfer of a pattern in a resist layer, e.g., a rastered pattern, to the metal imprinting surface of a stamper/imprinter. However, separation of the stamper/imprinter from the mechanically fragile, patterned resist layer of the “master” frequently incurs damage to the resist surface, which damage disadvantageously results in the “master” being unusable for making additional stampers/imprinters. Thus, it is common practice to make a “family” of stampers/imprinters, i.e., wherein a first, or “father” stamper/imprinter is formed from the master and then used for making a “mother” stamper/imprinter, and then the “mother” stamper/imprinter is used for making at least one “son” stamper/imprinter. However, the additional replication steps involved in making the “family” of stampers/imprinters typically results in a loss of fidelity of the pattern which is to be replicated in the recording medium.
Another drawback/disadvantage associated with the use of a “master” comprising a patterned resist layer is that the depth of the pattern features formed in the resist layer is constrained, i.e., limited, by the process utilized for defining the pattern in the resist layer. More specifically, the thickness of the photoresist layer determines the maximum depth of the pattern features. Therefore, if deeper features are desired, resist thickness must be increased. However, such increase in resist thickness incurs a disadvantage in that an increased exposure dose is required for patterning therein, leading to an increase in the pattern recording time.
In view of the foregoing problems attendant upon the use of a conventional “master” with its mechanically fragile resist layer for pattern definition in the stamper/imprinters manufactured therefrom, there exists a need for means and methodology for manufacturing stampers/imprinters for recording media patterning which are free of the above-described problems, drawbacks, and disadvantages associated with the use of conventional “masters” for manufacture of stampers/imprinters for recording media patterning. Moreover, there exists a need for methodologies and instrumentalities which facilitate rapid, cost-effective patterning of various types of recording media, including, but not limited to, formation of servo patterns in magnetic and magneto-optical (MO) recording media.
The present invention addresses and solves the aforementioned problems, drawbacks, and disadvantages associated with the use of conventional “masters” for manufacture of stampers/imprinters for recording media patterning, while maintaining full compatibility with the requirements of automated manufacturing technology.